Monday, September 24, 2012

New paper shows how natural variability controls North Atlantic climate

A new paper published in Nature Geoscience reconstructs the North Atlantic Oscillation, a natural climate phenomenon that is the "dominant mode of atmospheric variability" of the North Atlantic region. The authors find that over the past 5,200 years, when the North Atlantic Oscillation (NAO) index is positive, "Europe and the eastern US are mild and wet, whereas Greenland and northern Canada are cold and dry. A negative index is associated with the reverse pattern." The authors also show that modern measurements of the summer NAO index have been strongly negative since about 2005, and thus, Greenland and northern Canada would be expected to be relatively warm and wet relative to Europe and the eastern US. Once again, natural climatic variations such as the NAO have been shown to dominate climate, not greenhouse gases.

Summer NAO index (June, July, August or JJA) shown in upper left graph has been strongly negative since ~2005.

NAO index over the past 5,200 years shown in 3rd graph from right with positive index in red and negative in blue. "Greenland warm relative to Europe" shown in far right graph.

Article tool

Climate in the Arctic region and northwestern Europe is strongly affected by the North Atlantic Oscillation1, 2 (NAO), the dominant mode of atmospheric variability at mid-latitudes in the North Atlantic region. The NAO index is an indicator of atmospheric circulation and weather patterns: when the index is positive, Europe and the eastern US are mild and wet, whereas Greenland and northern Canada are cold and dry. A negative index is associated with the reverse pattern. Reconstructions of the NAO have so far been limited to the past 900 years3. Here we analyse a 5,200-year-long, high-resolution lake sediment record from southwestern Greenland to reconstruct lake hypolimnic anoxia, and link the results to an existing reconstruction of the NAO index from tree rings and speleothems3. Using the relationship between the two records, we find that around 4,500 and 650 years ago—around the end of the Holocene Thermal Maximum and the beginning of the Little Ice Age, respectively—the NAO changed from generally positive to variable, intermittently negative conditions. We suggest that variability in the dominant state of the NAO tend to coincide with large-scale changes in Northern Hemisphere climate. However, the onset of the Medieval Climate Anomaly was not associated with any notable changes in the NAO.